We know that the typical American diet — filled with processed food and added sugar — is making us fat. But it’s also making us depressed, according “The Happiness Diet,” a new book that links food to feelings.

“Your brain is made of food, and the right foods are the foundation of good mental health,” said Drew Ramsey, M.D., who co-authored “The Happiness Diet” with veteran health editor, Tyler Graham. “You can't feel your best if you starve the brain.”

Undernourished brains, the authors say, go hand-in-hand with overweight bodies — and they back up these claims with voluminous amounts of data. Thanks to big agriculture and the food industry, our grocery stores are full of food loaded down with added sugar, refined carbohydrates and vegetable fats, like corn oil. Our refrigerators contain factory-farmed meat and dairy products.

These foods, the authors say, are stripped of vital nutrients, such as magnesium and vitamin B12, and essential fats, such as conjugated linoleic acid. You’ll find these natural mood-boosters in whole grains and leafy greens, but also in grass-fed beef and full-fat dairy, which dieters tend to avoid for fear of gaining weight.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

“I do believe (that) much of the food available to us is nutrient-poor, and that most Americans eat a largely unhealthful diet and need to get back to eating real food,” wrote Elisa Zied, R.D., in an e-mail. But calories do matter, and portions consumed matters, too. “That’s really the bottom line when it comes to sustainable weight loss,” she said.

Exercise is also key to weight loss — something the authors don’t address in “The Happiness Diet.” Though exercise is Ramsey’s “top prescription for anxiety” when working with patients, he and Graham point out that if you’re eating the typical American diet, you don’t feel much like exercising. Fill your plate with kale and wild-caught salmon and you’re bound to feel better — and ready to tackle the treadmill.

OK, so whole foods make you feel better, and should make you skinnier. But don’t they cost a whole lot more?

“The biggest myth out there is that eating right is expensive,” said Ramsey. If you're really into this kind of thing, you could skip buying cuts of meat at the grocery store; instead, gather some like-minded buddies to buy a whole animal from a local farmer. He also suggests joining a CSA (community-supported agriculture) program for a weekly supply of produce, which is usually less expensive than what you’d get at the market. And freeze, freeze, freeze — everything from fish to fruit to butter.

Spending money on good food is worth it, said Graham, who pointed out that 100 years ago, people spent much more on food than they do now. “Your brain is made out of food,” he said. "What's more important, having 200 cable channels or feeding your brain the nutrients it needs?"